This invention relates to tapered roller bearings and more particularly to a method of adjusting tapered roller bearings with considerable precision.
Tapered roller bearings have the capability of carrying high radial loads as well as thrust loads, and furthermore when they are utilized in pairs, one bearing may be adjusted against the other to obtain the desired amount of end and radial play. For these reasons tapered roller bearings arranged in pairs are utilized almost universally in the nondriven wheels of automobiles manufactured in the United States. Normally, the non-driven wheels are the front wheels.
In the typical front wheel mounting (see FIG. 1), the wheel is bolted to a hub which in turn is mounted on a spindle by a pair of tapered roller bearings. The cups of the bearing are pressed through the bores within the hub until the cup back faces come against the shoulders. The cone assemblies, that is, the cones with the rollers and cages surrounding them, fit loosely over the spindle. The back face of the inboard cone bears against a cone backing shoulder on the spindle, while the back face of the outboard cone is located by a nut which threads over the end of the spindle. The nut affords the adjustment, and it should be run up until the bearing mounting is between 0.001 and 0.008 inches end play, a range of 0.007 inches.
On present automotive assembly lines, the bearing adjustment is a manual operation which on occasion does not receive the skill and care it demands. As a result, some bearings could be released with too much preload, while others could be set with too much end play. Either may be corrected by the dealer, but such corrections are a nuisance and increase the cost of maintaining warranties.
Because of the large number of dimensions which affect the bearing adjustment for a non-driven wheel, it has been impractical to maintain each of the dimensions within acceptable tolerances so that the overall tolerance in the bearing assembly falls within the acceptable service range, that is, 0.001 to 0.008 inches end play. In this regard, each of the following dimensions, or more specifically the tolerances allowed for those dimensions, affects the bearing adjustment, that is, those tolerances determine the position to which the spindle nut must be advanced to provide the proper bearing adjustment:
1. The axial dimension a between the backing shoulders of the hub, PA1 2. The cup back face location b.sub.1 and b.sub.2 for the inboard and outboard bearings, respectively each of which is basically the distance between a known working diameter along the cup raceway and the cup back face when the cup is not stressed (out of its hub bore), PA1 3. The cone back face locations c.sub.1 and c.sub.2 for the two bearings each of which is basically the distance from the known working diameter on the outer tapered surface of the rollers and the cone back face. PA1 4. The cup diameters d.sub.1 and d.sub.2 for the two cups (when the cups are pressed into the bores of the hub, they shrink and this reduces the raceways thereof, causing the cones to project further beyond the cups), and PA1 5. The hub bore diameters e.sub.1 and e.sub.2 for receiving the two cups. From the foregoing, it is apparent that the adjusting nut on the spindle compensates for nine tolerances.
The nine tolerances result in an overall tolerance range of 0.040 inches in the typical bearing assembly. However, according to the laws of probability, when parts are selected randomly, not all parts will have tolerances at the high side or the low side, but on the contrary the parts will constitute a mixture of high and low tolerances. Because of the variation in tolerances, the axial dimension of a substantial number of the assembled bearings will fall generally midway between the extremes of the overall tolerance range, and this establishes a mean axial dimension. Even so a substantial number of the bearing assemblies will deviate from the mean axial dimension and by a relatively large amount. More specifically, the laws of probability tell us that when standard tolerances are held in the nine dimensions previously discussed, 68.26% of the bearing assemblies will fall within a range of 0.006 inches, 95.46% will fall within a range of 0.012 inches, and 99.73% will fall within a range of 0.018 inches. All but the first of these ranges are outside the acceptable service range of a tapered roller bearing assembly for automotive applications, which is 0.007 inches.
Of course, the overall tolerance range within the bearing assembly may be reduced by decreasing the nine individual tolerances which contribute to the overall tolerance. This, however, is extremely expensive.
Thus, from the foregoing, it is apparent that under current techniques, the only practical device for accommodating the large overall tolerance in a bearing assembly is the adjusting nut, even though the adjustment afforded by such a nut is dependent on the judgment of the operator who installs it.
Tapered roller bearings are also employed in mounting for the front wheels of front wheel drive automobiles. In these applications even more tolerances are involved and the adjustment required for satisfactory performance is quite critical. In particular, the two bearings are normally set quite close together so that the universal joint may be located as close to the steering knuckle pivot axis as possible. This enables the universal joint to transmit torque as smoothly as possible. Due to the relatively small spread between the bearings, end play within the bearing assembly produces considerably more wheel wobble. Stated differently, as the bearing spread decreases, tire wobble increases for any given amount of end play. Consequently, the bearing adjustment is quite critical. To increase the rigidity of such mountings as much as possible, the cones of the two bearings are usually press-fitted over a hub sleeve to which the drive shaft is splined. Also, press-fitted cones are desirable for applications having rotating cones. Hence, tolerances of the two cone bores as well as the tolerance of the hub sleeve also affect the overall tolerance of the bearing assembly and indeed increase the overall tolerance range.